Fix layer-mask NoDataInBounds at low zoom (MD mode)

[scottstanie]

Bug

When a layer mask is active and the user zooms out so the entire raster fits the viewport, the velocity overlay disappears and the backend logs:

File "/.../titiler/core/factory.py", line 959, in tile
    with self.reader( …
File "<attrs generated methods rio_tiler.io.xarray.XarrayReader>", line 36, in __init__
File "/.../rio_tiler/io/xarray.py", line 84, in __attrs_post_init__
    self.bounds = tuple(self.input.rio.bounds())
File "/.../rioxarray/rioxarray.py", line 1006, in _internal_bounds
    raise NoDataInBounds(
rioxarray.exceptions.NoDataInBounds: Unable to determine bounds from coordinates. Data variable: velocity

At high zoom the layer renders fine — symptom only shows once the requested tile is at a low enough zoom that dataset_for_tile_zoom(tile_z) returns a coarsened pyramid level.

Root cause

_apply_layer_masks_md looked up mask variables in state.dataset (level 0):

mask_da = state.dataset[var]
…
da = da.where(mask_da >= threshold)

But the tile-rendering caller in XarrayPathDependency resolves da from target.dataset_for_tile_zoom(tile_z), which returns a coarser Dataset when zoomed out. da.where(mask_da) then outer-aligns two y/x grids whose cell centres don't coincide. The resulting coordinates are interleaved and non-monotonic — rioxarray's _internal_bounds() can't extract a uniform resolution from them and raises.

reproject_match (used by the custom-mask path) doesn't apply here because the layer-mask path went straight through where.

Fix

Added source_ds kwarg to _apply_layer_masks_md. Tile-rendering callsite now passes the same pyramid-level Dataset that da came from. The other three callers (/point, /buffer_timeseries, /trend_analysis) all work on state.dataset directly and keep relying on the default — they're at level 0 already, so no behavior change.

Test plan

• ruff / ruff-format / mypy pass
• Reviewer to verify on the affected dataset (the test box I had loaded was COG mode, not MD — different code path; couldn't reproduce locally without a Zarr cube). Repro: load a --stack-file cube.zarr, apply a closure_coherence ≥ 0.5 mask, zoom out until the whole raster is in view. Before this PR: overlay vanishes + 500s in log. After: overlay continues to render at every zoom.

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